Hormone therapy for breast cancer is a treatment for breast cancers that are sensitive to hormones. Some forms of hormone therapy for breast cancer work by blocking hormones from attaching to receptors on cancer cells. Other forms work by decreasing the body's production of hormones.

Hormone therapy is only used for breast cancers that are found to have receptors for the naturally occurring hormones estrogen or progesterone.

Hormone therapy for breast cancer is often used after surgery to reduce the risk that the cancer might return. Hormone therapy also may be used to shrink a cancer before surgery. If the cancer shrinks, it may be possible to remove less breast tissue during surgery. Using hormone therapy before surgery also gives your health care team information about how well your cancer responds to this treatment.

If your cancer has spread to other parts of your body, hormone therapy for breast cancer may help control it.

Hormone therapy for breast cancer is only used to treat cancers that are hormone sensitive. Hormone-sensitive breast cancers are fueled by the natural hormones estrogen or progesterone.

A breast cancer that's sensitive to estrogen is called estrogen receptor positive, also called ER positive. A breast cancer that's sensitive to progesterone is called progesterone receptor positive, also called PR positive. Many breast cancers are sensitive to both hormones.

Tests in a lab can show whether the cancer cells have receptors for estrogen or progesterone. If at least 1% of the cells have receptors, you can be considered for hormone therapy. These tests help your health care team understand how to treat your breast cancer.

Hormone therapy for breast cancer can help to:

• Prevent cancer from coming back.
• Reduce the size of a cancer prior to surgery.
• Slow or stop the growth of cancer that has spread.
• Decrease the risk of cancer developing in other breast tissue.

Side effects of hormone therapy for breast cancer are different for each medicine. Side effects of the most common medicines include:

Tamoxifen

• Hot flashes.
• Night sweats.
• Vaginal discharge.
• Irregular periods in premenopausal women.
• Fatigue.

Aromatase inhibitors

• Joint and muscle pain.
• Hot flashes.
• Night sweats.
• Vaginal dryness or irritation.
• Fatigue.
• Impotence in men with breast cancer.

Less common, more serious side effects of hormone therapy may include:

Tamoxifen

• Blood clots in veins.
• Cataracts.
• Endometrial cancer or uterine cancer.
• Stroke.

Aromatase inhibitors

• Heart disease.
• Thinning bones.

There are several approaches to hormone therapy.

Medicines that block hormones from attaching to cancer cells

One approach to hormone therapy is to stop the hormones from attaching to the receptors on the cancer cells. When the hormones can't access the cancer cells, the cancer's growth may slow and the cells may die.

Breast cancer medicines that have this action include:

• Tamoxifen. Tamoxifen is usually taken daily in pill form. It's often used to reduce the risk of cancer recurrence after treatment for early-stage breast cancer. In this situation, it's typically taken for 5 to 10 years.

  Tamoxifen also may be used to treat advanced cancer. In women, tamoxifen is an option for those who have been through menopause and those who haven't. In men, tamoxifen is usually the first medicine considered for hormone therapy.

• Toremifene (Fareston). Toremifene is taken as a daily pill. It's used to treat breast cancer that has spread to other areas of the body. In women, it's approved in those who've been through menopause.
• Fulvestrant (Faslodex). Fulvestrant is given as a shot every month after first getting a dose every two weeks for the first month. It's used to treat breast cancer that has spread to other parts of the body. In women, it's only used in those who've been through menopause.

Medicines that stop the body from making estrogen

Aromatase inhibitors are medicines that reduce the amount of estrogen in the body. This medicine deprives breast cancer cells of the hormones they need to grow.

In women, aromatase inhibitors are only used in those who have gone through menopause. They cannot be used unless your body is in natural menopause or in menopause induced by medicines or removal of the ovaries. In men, aromatase inhibitors are typically used with other medicines to better block hormones in the body.

Aromatase inhibitors used to treat breast cancer include:

• Anastrozole (Arimidex).
• Exemestane (Aromasin).
• Letrozole (Femara).

These medicines can reduce the risk of cancer recurrence in those who have been treated for early-stage breast cancer. Aromatase inhibitors also can be used to treat advanced breast cancer. And they may be an option for people who no longer benefit from tamoxifen treatment or whose treatment is completed.

Aromatase inhibitors are pills you take once a day. All three aromatase inhibitors work the same way and reduce the production of estrogen in the body.

How long you continue aromatase inhibitors depends on your specific situation. Current research suggests at least five years of hormone therapy. Some people may benefit from 7 to 10 years of treatment. You and your health care provider can work together to decide how long you should take them.

Treatments to stop ovarian function in premenopausal women

Women who haven't gone through menopause, either naturally or as a result of cancer treatment, can have treatment to stop their ovaries from producing hormones.

Options may include:

• Medicines, such as goserelin (Zoladex) or leuprolide (Lupron Depot).
• Surgery to remove the ovaries, called oophorectomy.
• Radiation therapy aimed at the ovaries.

Treatments to stop ovarian function may allow those who haven't been through menopause to take medicines only available to those who've been through menopause.

Combining targeted therapy with hormone therapies

Hormone therapy is sometimes combined with targeted therapy. Targeted therapy medicines attack specific chemicals in cancer cells. The combination can make hormone therapy more effective.

Medicines used in this way include:

• Abemaciclib (Verzenio).
• Alpelisib (Piqray).
• Palbociclib (Ibrance).
• Ribociclib (Kisqali).
• Everolimus (Afinitor).

You'll meet with your cancer doctor, called an oncologist, regularly for follow-up visits while you're taking hormone therapy for breast cancer. Your oncologist will ask about any side effects you're experiencing. Many side effects can be controlled.

Hormone therapy following surgery, radiation or chemotherapy has been shown to reduce the risk of breast cancer recurrence in people with early-stage hormone-sensitive breast cancers. It also can effectively reduce the risk of metastatic breast cancer growth and progression in people with hormone-sensitive cancers.

Depending on your circumstances, you may undergo tests to monitor your medical situation. These tests help watch for cancer recurrence or progression during hormone therapy. Results of these tests can give your oncologist an idea of how you're responding to the treatment. Your treatment plan may be adjusted accordingly.